A numerical control system is available having a counter for storing a machine position by counting pulses, which are generated by a pulse coder, in dependence upon direction of rotation, which counter continues to store the machine position even if power is cut off. In a numerical control system which includes an absolute pulse coder comprising such a pulse coder and counter, if a reference point return operation is performed just once at the time of machine installation, then automatic operation can start immediately without requiring that the reference point return operation be performed each time power is introduced to the system thereafter. In other words, in an NC system which performs numerical control using only the machine position and not a commanded position (a position conforming to commanded pulses) at the time of NC control, an absolute pulse coder is very effective since it is unnecessary to perform a reference point return operation.
However, in a numerical control system which uses a commanded position in addition to the machine position, it is necessary that the commanded position be restored. Though no difficulties are encountered if the machine position and commanded position are equal, these positions are usually different. The reason for this is that the movable machine element is moved by the commanded pulses and the position thereof is corrected by various types of correction processing, such as for a pitch error correction and straightness correction.
Accordingly, an object of the present invention is to provide a commanded position restoration method in a numerical control system, through which a commanded position which conforms to a machine position at the time power is introduced can be obtained simply and accurately.